The first electronic speckle-pattern interferometry (ESPI) systems were developed in the early 1970s. ESPI has since been applied to study many different problems, primarily in research laboratories. Moreover, most ESPI systems are out-of-plane displacement sensitive systems. Such applications include nondestructive inspection (NI), vibration analysis, materials characterization, electronic packaging study, biomechanics study, and verifications of the results from finite element analysis. ESPI has also been used to detect unbonded areas of adhesive joints, to study vibration in the automotive industry, to measure the thermal expansion of a piston, and to monitor strains and crack-propagation paths of rocks in fracture mechanics.
Young's modulus can be determined from FEM by using an iterative approach with the results from ESPI (both dynamic mode and static mode) and LDV. In-plane sensitive ESPI has been used for analyzing thermal strain on ceramic and composite materials, for conducting fracture mechanics testing, and for measuring the in-plane strain on high-speed rotating components.
The non-destructive testing/evaluation (NDI) methods most widely used in industry include ultrasonics, eddy-current measurement, and x-radiography. The ultrasonic techniques are used to detect flaws by measuring the response to an ultrasonic stress wave. However, due to the point-by-point or line-by-line scanning procedures involved, the ultrasonic method is typically slow. A medium, such as water or gel, is usually required to transfer the ultrasound energy from the transducer into the material, which is inconvenient in some cases.
X-radiography relies on the differential absorption or scattering of x-ray photons as they pass through a material. Flaws that either allow more x-ray photons to pass or that absorb or scatter the photons can be imaged if the effect is sufficiently pronounced. The molecules in many polymer composites are usually of low atomic weight nuclei, and hence the absorption of x-rays is low and contrast is usually poor, especially for a thin plate. Eddy-current measurement is only applicable to metallic materials. Furthermore, none of these methods relate flaw detection to the stress/strain states of a test object in any fundamental way. Therefore, how a detected flaw affects the performance of a particular component cannot be revealed by current detection processes.
Ultrasonic and x-ray technologies are good at determining the geometry and detailed location of the flaws, especially the internal flaws in a structure. Unfortunately, neither of these methods relate the detection to the effect of the flaws on the material or strength. Furthermore, for inspection of a large area, these techniques are slow and costly. Although ESPI is most sensitive to surface and subsurface flaws, internal flaws can also be detected from their induced "disturbances" on the surface when an appropriate stressing technique is used.
Among all NDI techniques discussed, laser interferometry, including holographic interferometry, ESPI, and shearography are the only tools which can detect flaws through direct measurement of a material's strength-related parameters such as deformation/displacement or strain. These techniques offer the opportunity for directly assessing the actual effects of the detected damage on the structures. Laser interferometry techniques are highly sensitive to a wide variety of flaws and can inspect a large area at fast speed.
In contrast to traditional methods, loading (stressing) is an essential part of NDI processes based upon ESPI. The loading provides an important connection between the detected flaws and the effect of the flaws on the integrity and strength of the structures under test. However, ESPI systems based on in-plane displacement sensitive optical configurations have not been utilized to the same extent as out-plane-displacement sensitive configurations. One reason is that the in-plane systems are bulky and difficult to align. There remains an outstanding need, therefore for a lightweight and/or portable ESPI system based upon an in-plane displacement sensitive optical configuration.